Miss C. A. Raisin—Rocks from Socotra. 505 
pressure,! or the introduction of water.2 If the change in the 
condition of the rock were rapid, a contraction might result, and 
might perhaps develope along a curve, a layer of inclosures, as in 
the effects from pressure demonstrated by Prof. Judd.? The strain 
increased, the curve might become a crack, so that the corroding 
magma could make its way along the weakened surface, and cause the 
The darkened part represents devitrified ground-mass including hornblende flakes. 
The dots are minute enclosures. ‘The porphyritic quartz is left clear. 
interrupted embayments which I have mentioned. The degradation 
of a crystal may be connected with planes of weak cohesion, as seems 
illustrated by a grain, which shows very faint, almost concentric 
zoning; corrosion has originated 
at seven or eight points along the 
border of the crystal, and, as it 
reaches successive zones, has be- 
gun to spread along them, result- 
ing in a form like three or four 
shallow saucers placed one above 
another (Fig. 2). 
(2). Orthoclase-Felsite. The 
hand-specimen is dull, and of a 
very pale chocolate-brown, with 
slight traces of fluidal structure. 
The rock is microporphyritic, 
crowded with felspars of definite 
crystalline form, which are de- 
composed, and contain brightly- 
polarizing films. From their form, and the occasional occurrence of 
1 Tsch. Min. Mitt. Bd. viii. p. 421; A. Lagorio, Abs. in Min. Mag. 1887. 
British Petrography, Mr. Teall, p. 407. 
2 Grou. Mac. Jan. 1888, pp. 10, 11, Lavas of Krakatoa, Prof. Judd; Pres. 
Address Geol. Soc. 1885, p. 54, Prof. Bonney. 
8 Q.J.G.S, 1885, p. 376, On Peridotites of Scotland. 
